Aluminum nitride (AlN) crystal has received attention as a substrate material for optoelectronic and other semiconductor devices because it has a broad energy bandgap of 6.2 eV, a high thermal conductivity of about 3.3 WK−1 cm−1, and high electrical resistance.
As means to grow aluminum nitride crystal as such, the vapor-phase technique of sublimation deposition is for example employed. Growth of aluminum nitride crystal by sublimation deposition is carried out by the following steps for example. Namely, a polycrystalline aluminum nitride source material is placed in the lower part of a growth chamber, and a starting substrate is set onto a susceptor in the upper part of the growth chamber, so that the substrate and the polycrystalline aluminum nitride source material face each other. The polycrystalline aluminum nitride source material is then heated to a temperature at which the polycrystalline aluminum nitride source material sublimates. The heating sublimates the polycrystalline aluminum nitride source material, producing sublimation gases, and monocrystalline aluminum nitride grows onto the surface of the starting substrate emplaced where the temperature is lower than that of the polycrystalline aluminum nitride source material.
Thus, between the starting substrate and the polycrystalline aluminum nitride source material there is a temperature gradient (temperature difference) where the temperature drops heading from the starting substrate to the polycrystalline aluminum nitride source material. Therefore, between the starting substrate and the susceptor also there is a temperature gradient where the temperature drops heading from the starting substrate to the susceptor, and within the starting substrate as well there is a temperature gradient where the temperature drops heading from the surface opposing the polycrystalline aluminum nitride source material to the surface opposing the susceptor. If between the starting substrate and the susceptor a gap appears, the back side of the starting substrate will incur exposure to the aluminum-nitride-crystal growth atmosphere, whereby due to the just-noted temperature gradients the atoms constituting the starting substrate will sublimate and end up recrystallizing from the starting substrate onto the susceptor, and from high-temperature areas into low-temperature areas within the starting substrate. Progression of this sublimation and recrystallization leads to the occurrence of holes penetrating the starting substrate. Should the sublimation progress further, it can happen that atoms constituting the aluminum nitride crystal grown onto the starting substrate are transported to low-temperature areas of the starting substrate, susceptor, etc. Holes can as a consequence occur in the aluminum nitride crystal grown onto the front side of the starting substrate. Accordingly, preventing sublimation of starting substrates to minimize occurrence of holes in the starting substrates has become a subject of concern.
As technology for thus preventing sublimation of a starting substrate, Japanese Unexamined Pat. App. Pub. No. 2006-290676 (Patent Document 1), for example, describes employing an alumina-based adhesive for high temperatures to adhere the starting substrate to the susceptor. The high-temperature adhesive is said to demonstrate sufficient strength even at high temperatures of 1000° C. or more.
Further, Japanese Unexamined Pat. App. Pub. No. H11-510781 (Patent Document 2), describes a coating, consisting of a metal and metal compound, being formed on the back side of a starting substrate.
In addition, Japanese Unexamined Pat. App. Pub. No. 2005-247681 (Patent Document 3) describes a process in which a metal material is disposed atop a graphite pedestal, a starting substrate is arranged on the metal material, and a pressing member is placed onto the starting substrate, and with the pressing member, at a high temperature of at least 1700° C., a high pressure of not more than 87.5 kPa is applied to the graphite pedestal, metal material, and starting substrate to fix and unify them. The metal material is stated to be at least one material selected from titanium, vanadium and zirconium.
Japanese Unexamined Pat. App. Pub. No. H09-268096 (Patent Document 4), meanwhile, describes coating with a protective layer the surface(s) of a starting substrate other than the face on which single crystal is grown. The protective layer is stated to be made of at least one substance selected from tantalum, tungsten, niobium, molybdenum, rhenium, osmium, iridium, and their carbides, borides and nitrides. Employing an adhesive to affix the protective layer to the pedestal is also disclosed.    Patent Document 1: Japanese Unexamined Pat. App. Pub. No. 2006-290676    Patent Document 2: Japanese Unexamined Pat. App. Pub. No. H11-510781    Patent Document 3: Japanese Unexamined Pat. App. Pub. No. 2005-247681    Patent Document 4: Japanese Unexamined Pat. App. Pub. No. H09-268096